1. Field of the Invention
The invention relates to a photoplotting method and an arrangement for plotting a computer-stored raster image on a plane, photosensitive record carrier, i.e., for plotting picture elements (pixels) on a record carrier. The picture elements to be plotted may be arranged, at raster (grid pattern or matrix) positions, according to a raster, which may be defined by points of intersection of equidistant X/Y lines.
The photoplotting method and arrangements serve, e.g., to produce photographic artwork for printed circuit boards.
2. Discussion of Background Information
Photoplotters plotting a computer-stored raster image on a plane, photosensitive record carrier are known in the prior art (information sheet "Xenon Plotter" by the company MIVATEC, Hard- und Software GmbH, Benzstr. 17, D-71171 Schonaich, of Oct. 15, 1996).
During said plotting, an exposure head (with a plotting area of about 5.times.7.5 mm) is guided over a record carrier, which has a large area (up to about 720.times.600 mm) in relation to it, for the purpose of plotting picture elements, which make up the raster image of the record carrier.
The exposure head is provided with an LCD (liquid crystal device) matrix (or liquid crystal array) of light modulation elements. This LCD matrix comprises multiple liquid crystal elements arranged in form of a matrix, each of which can be selectively controlled by electrical means in order to open or block it for transmitted light.
If transmitted light is projected onto this LCD matrix, it can only pass as a "light beam" through the matrix at the those element points that are controlled to admit passage. Each of the light beams admitted forms a corresponding picture element on the photosensitive record carrier. Missing light beams--equivalent to blocked elements--form "missing picture elements".
The raster image of the record carrier comprises a multiplicity of picture elements, including plotted or non-plotted (missing) ones.
In the aforesaid arrangement, light from a Xenon flash lamp is projected onto the LCD matrix. Since the exposure head containing the LCD matrix has a much smaller area (to be compared with the square of a chessboard) than the record carrier (to be compared with a chessboard with an enormous number of squares), said exposure head must successively be guided to all the positions of the chessboard squares so that picture elements can be plotted there.
In view of the desired high resolution (in the range of &gt;10 micrometers) of the raster image of the record carrier, the movement of the exposure head over the entire area of the record carrier imposes the highest tolerance requirements with regard to the precision of the movement mechanism for the movement of the head.
The greater the movement distances, the more difficult it is to fulfil the highest tolerance requirements. The movement of the exposure head over the entire area of the record carrier is therefore problematic and limits the accuracy of plotting, quite apart from the enormous equipment and cost expenditure for such a movement mechanism. Additionally, the aforesaid photoplotters involve long plotting times (6 minutes to 1 hour, depending on resolution).